1. Field of the Invention
The invention disclosed herein relates to nuclear instrumentation and, in particular, to a neutron instrument useful for geologic evaluations.
2. Description of the Related Art
In the quest for petrochemical materials, such as oil and gas, many techniques are used to evaluate sub-surface materials. Typically, the sub-surface materials are evaluated by drilling boreholes into the earth and performing measurements with various instruments. Performing measurements is commonly referred to as “well logging.”
Well logging is a technique used to take measurement properties of the sub-surface materials from the boreholes. In one embodiment, a “logging instrument” is lowered on the end of a wireline into a borehole. The logging instrument sends data via the wireline to the surface for recording. In some other embodiments, logging is performed while drilling, by use of instrumentation included within a drill string.
Measuring naturally occurring gamma radiation (or gamma rays) is one way to determine characteristics of the sub-surface materials. Accordingly, a gamma radiation detector may be used as a component of the logging instrument.
Gamma rays may be emitted from various sub-surface materials. For example, gamma rays may be emitted from the various formation layers and from borehole materials such as mud. In particular, the mud may contain natural emitters of gamma radiation, such as potassium. As the gamma radiation detector moves along the borehole, gamma rays from the various sources may enter the gamma radiation detector and be detected.
The gamma radiation detector may be used to estimate the energy of each gamma ray entering the detector. The gamma ray energy levels may be used to create gamma ray energy spectra. The gamma ray energy spectra reflect types and quantities of elements (referred to as elemental yields upon separation) in the formation layers and the borehole materials.
Other techniques may take advantage of measurements of gamma rays. For example, a neutron source may be used to activate the various sub-surface materials in the environment of and surrounding the borehole. Neutron activation results in additional gamma-ray emissions. Accordingly, neutron activation of sub-surface materials is useful for generating additional data descriptive of the sub-surface materials.
One instrument useful for performing neutron activation and activation analyses is the FLEXSM instrument, commercially available from Baker Hughes Incorporated, of Houston Tex. With the FLEX instrument, in general, a pulse of neutrons is used to activate nuclei of sub-surface materials. The neutrons interact with the nuclei, which may emit characteristic gamma rays, for the most part through one of two processes, neutron inelastic scattering, and neutron capture. The gamma rays from the neutron inelastic scattering occur during, or very soon after, the pulse of neutrons. The gamma rays resulting from the neutron capture events are typically delayed. The gamma rays may be detected with various gamma radiation detectors.
The gamma rays resulting from neutron inelastic interactions are used to create “inelastic gamma ray” energy spectra while delayed gamma rays emissions generally provide “capture gamma ray” energy spectra. Data from the two types of gamma ray energy spectra are used to deduce the elemental yields of the surrounding sub-surface materials.
Unfortunately, a significant number of background gamma rays are generated from the matter within the instrument itself. These background gamma rays interfere with the collection of data signals from the sub-surface materials. Although background gamma rays resulting from neutron capture can be reduced by applying a material, such as boron, to the outside of the housing of the instrument to prevent outside thermal neutron interacting with tool materials, no known material exists for the absorption of fast neutrons that interact with tool materials. Consequently, analysis of gamma ray spectra associated with neutron inelastic scattering becomes more difficult to analyze.
What are needed are techniques for correcting measurements of gamma radiation emitted from sub-surface materials, where the gamma radiation is produced by neutron inelastic scattering. In particular, the techniques are needed to compensate for gamma radiation resulting from interfering signals, such as gamma rays from within the survey instrument itself.